The present invention relates to metal alkoxides and to the preparation and isolation of metal alkoxides, and especially, to metal alkoxides that are liquid when pure and/or exhibit increased solubility in a wide variety of solvents, and to methods of preparation of such metal alkoxides.
Metal alkoxides, and particularly alkali metal alkoxides, are widely used in industry as catalysts and as stoichiometric reagents. These reagents are used in diverse reaction chemistries such as alkylation, isomerization, rearrangements, condensations, transesterifications and eliminations. See, for example, D. E. Pearson, C. A. Buehler Chemistry Reviews 74, 45 (1974).
As pure solid compounds, these materials are ionic in character as a result of the strongly electropositive nature of the metals. See, for example, D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides, Academic Press, London (1978). For derivatives of the same element, the covalent character of the metal-oxygen bond increases with the greater inductive effect of the alkyl group. For example, a tertiary butoxide has a higher covalent character than the corresponding primary n-butoxide. The trend in covalent character relative to the counter ion in the case of alkali metals, for example, is that lithium alkoxides are more covalent than sodium or potassium alkoxides. This phenomenon, coupled with steric factors, leads to a slightly greater inherent stability of the isolated solid tertiary alkoxides. Unfortunately, these caustic solids readily react with atmospheric water and carbon dioxide. Furthermore, these solid metal alkoxides are rather dusty, which can be problematic when handled on a large scale. Some of the primary alkoxides are also prone to spontaneous combustion in air. See Y. El-Kattan, J. McAtee, xe2x80x9cSodium Methoxidexe2x80x9d Encyclopedia of Reagents for Organic Synthesis, 4593, Ed. L. A. Paquette, John Wiley and Sons, NY (1995).
To provide a safer material, metal alkoxides are often dissolved in a solvent. In general, liquids are, for example, easier to transfer from drums or cylinders into reactors (reducing the exposure of human handlers to dangerous materials), more easily kept under an inert atmosphere, and provide more options for modes of addition to the substrate. Unfortunately, alkali metal alkoxides and other metal alkoxides exhibit only rather low solubility in the alcohols from which the alkoxides are made, usually in the range of 2-25 wt %. For example, sodium isopropoxide is only soluble up to about 2 wt % in isopropanol. The low solubilities of many alkoxides have been attributed to the ionic character and the extent of oligomerization or polymerization in solution. Another factor affecting solubility in an alcohol solvent is the propensity of alkoxides to form insoluble alcoholate complexes with the alcohol. Metal alkoxides are somewhat more soluble in polar ethereal solvents such as tetrahydrofuran and the polyethers (glymes). However, even in ethers, the solubility is generally less than 50%, especially at or below room temperature (that is, at or below 25xc2x0 C.). Moreover, the range of polar solvents is somewhat limited as a result of the reactivity of the alkoxide. Furthermore, in some cases the solvent of choice for the desired reaction involving a metal alkoxide is not compatible with the alkoxide or the metal alkoxide is insoluble therein.
It is very desirable to develop metal alkoxide reagents that facilitate the diverse reactions in which those reagents are use.
The present invention provides generally a method to produce relatively highly concentrated solutions of metal alkoxides in a wide variety of solvents. Solvents suitable for use in the present invention include aliphatic and aromatic hydrocarbons, and polar aprotic solvents such as dimethylformamide (DMF) and ethers. Preferably, the solubility of the metal alkoxide in the solvent is at least approximately 25 wt %. More preferably, the solubility of the metal alkoxide in the solvent is at least approximately 50 wt %. Most preferably, the solubility of the metal alkoxide in the solvent is at least approximately 75 wt %. These solubilities are achievable at relatively low temperature. Preferably, for example, these solubilities are exhibited in a temperature range of approximately xe2x88x9240xc2x0 C. to approximately 50xc2x0 C. More preferably, these solubilities are exhibited in a temperature range of approximately xe2x88x9225xc2x0 C. to approximately 25xc2x0 C. Most preferably, these solubilities are exhibited in a temperature range of approximately 0xc2x0 C. to approximately 25xc2x0 C. Surprisingly, the relatively high solubilities of the present invention are achievable even in aliphatic hydrocarbons and aromatic hydrocarbons.
The present invention also provides for isolation and characterization of the first pure liquid alkali metal alkoxide reagent and other liquid metal alkoxide reagents. As used herein, the terms xe2x80x9cpurexe2x80x9d or xe2x80x9cneatxe2x80x9d refer to a liquid having a purity of at least approximately 97 wt % (that is, the liquid is at least 97% metal alkoxide by weight). The purity is more preferably at least approximately 98 wt %. Most preferably, the purity is at least approximately 99 wt %. Unlike current metal alkoxide reagent compositions, the neat, liquid alkoxide reagents of the present invention are highly miscibile in all proportions with a wide variety of solvents, including, for example, aliphatic hydrocarbon solvents such as hexane and heptane or aromatic hydrocarbon solvents. Other suitable solvents include ethers and polar aprotic solvents. Furthermore, the compositions of the present invention are relatively easy to handle or transport. Moreover, the highly concentrated and/or neat liquid metal alkoxide reagents of the present invention allow higher reactor loading than is possible with current compositions, thereby maximizing productivity.
In one aspect, the present invention provides a method for synthesizing highly soluble metal alkoxides comprising the step of: reacting a tertiary alcohol with at least a stoichiometric amount of a metal reagent. Preferably, the reaction proceeds for a period of time sufficient for the reaction to go to completion. The metal reagent is preferably a group I metal, a group II metal, zinc, a metal alloy of a group I metal, a metal alloy of a group II metal, a metal alloy of zinc (suitable metal alloys, include, for example, NaK, NaHg or KHg), a compound of a group I metal, a compound of a group II metal or a compound of zinc (suitable, metal compounds include, for example, LiH, NaH, KH, Et2Zn or Bu2Mg) Preferred metals for use in the present invention include K, Li, Na, Cs, Mg, Ca or Zn. Likewise, metal alloys and metal compounds for use in the present invention preferably include K, Li, Na, Cs, Mg, Ca or Zn. In the case that a metal is used, the reaction preferably takes place above the melting point of the metal. Preferably, formation of a metalalcoholate complex is avoided. To avoid forming a metalalcoholate complex, an excess of metal reagent (for example, metal, metal alloy and/or metal compound) is preferably used.
Tertiary alcohols suitable for use in the present invention preferably have the general formula: 
(or HOCR1R2R3) wherein R1, R2, and R3 are, independently, the same of different, an alkyl group, an alkenyl group, an alkynyl group or an aryl group, and at least one of R1, R2, and R3 is a group of at least 3 carbon atoms. Preferably, at least on of R1, R2, and R3 is a branched group of at least 3 carbon atoms. More preferably, at least one of R1, R2, and R3 is a branched group of at least 6 carbon atoms. As used herein, the term xe2x80x9calkyl groupxe2x80x9d includes generally branched and unbranched alkyl group of the formula xe2x80x94CnH2n+1 (wherein n is an integer) and cyclic alkyl groups of the formula xe2x80x94CmH2m wherein m is an integer equal to or greater than 3. Alkyl groups preferably have 1 to 20 carbons. The term xe2x80x9calkenylxe2x80x9d refers generally to a straight or branched chain hydrocarbon group with at least one double bond, preferably with 2-20 carbon atoms, and more preferably with 3-10 carbon atoms (for example, xe2x80x94CHxe2x95x90CHR, xe2x80x94CH2CHxe2x95x90CHR, or xe2x80x94CH2CHxe2x95x90CHCH2CHxe2x95x90CHR, wherein R is, for example, H, an alkyl group, an alkenyl group, an alkynyl group or an aryl group). The term xe2x80x9calkynylxe2x80x9d refers to a straight or branched chain hydrocarbon group with at least one triple bond, preferably with 2-20 carbon atoms, and more preferably with 3-10 carbon atoms (for example, xe2x80x94Cxe2x89xa1CR, xe2x80x94CH2Cxe2x89xa1CR, or xe2x80x94CH2Cxe2x89xa1CCH2Cxe2x89xa1CR). The term xe2x80x9caryl groupxe2x80x9d preferably includes generally phenyl and napthyl groups. The term xe2x80x9cbranchedxe2x80x9d as use herein refers generally to a group that has at least one carbon atom attached to at least three other carbon atoms. Examples of branched groups include, but are not limited to, cyclic alkyl groups, aryl groups, arylalkyl groups and branched acyclic alkyl groups (for example, an isopropyl group). The alkyl, alkenyl, alkynyl and/or aryl groups of the present invention can be substituted or unsubstituted. Alkyl groups can, for example, be substituted with (that is, one or more of the hydrogen atoms thereof replaced with) an aryl group (making an arylalkyl group), an alkenyl group and/or an alkynyl group. Alkenyl groups can, for example, be substituted with an alkyl group and/or an aryl group. Alkynyl groups can, for example, be substituted with an alkyl group and/or an aryl group. Aryl groups can, for example be substituted with an alkyl group, an alkenyl group and/or an alkynyl group.
In another aspect, the present invention provides a solution of a metal alkoxide of the formula: 
(or R4nMOCR1R2R3), wherein R4 is an alkyl group, an aryl group or an alkoxyl group, M is a group I metal, a group II metal or zinc, and n is 0 or 1. M is preferably K, Li, Na, Cs, Mg, Zn or Ca. The concentration of metal alkoxide in the solvent is preferably greater than 50 wt %. More preferably, the concentration of the metal alkoxide is at least 75%. If M is a monovalent metal ion, n is 0. If M is a divalent metal ion, n is 1. Suitable solvents include aliphatic hydrocarbons, aromatic hydrocarbons, and polar aprotic solvents. As used herein, the term xe2x80x9calkoxyl groupxe2x80x9d refers generally to groups having the formula xe2x80x94OR5, wherein R5 is an alkyl group (substituted or unsubstituted). R5 can, for example, be xe2x80x94CR1R2R3.
In still another aspect, the present invention provides a compound having the formula: 
(or R4nMOCR1R2R3), wherein M, R1, R2, R3, R4 and n are as defined above. Unlike prior metal alkoxide reagents, the metal alkoxides of the present invention are liquid at or below 25xc2x0 C. and having a purity greater than approximately 97 wt %. Examples of such metal alkoxide compounds include, but are not limited to, potassium, sodium and lithium (3,7-dimethyl-3-octanoxide), .